Titanium-based carbonitride alloys, so called cermets, are today well established as an insert material in the metal cutting industry and are especially used for finishing operations. They generally comprise carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of a different composition. In addition to titanium, group VIa elements, normally both molybdenum and tungsten and sometimes chromium, are added to facilitate wetting between the binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, i.e., zirconium, hafnium, vanadium, niobium, and tantalum, are also added in all commercial alloys available today. All these additional elements are usually added as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase is normally a solid solution of mainly both cobalt and nickel. The amount of binder phase is generally 3-25 wt %. Other elements are sometimes added as well, e.g. aluminum, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase. Of course, commercially available raw material powders also contain inevitable impurities.
The most important impurity is oxygen. Oxygen has a high affinity for titanium. A normal impurity level for oxygen has historically been &lt;0.3 wt %. Recently, due to improved production methods for titanium-based raw materials, this level has been decreased to &lt;0.2 wt %, especially for grades with low nitrogen content. Very high oxygen levels are generally avoided since this may cause formation of carbon monoxide (CO) after pore closure during liquid phase sintering, which in turn leads to excessive porosity.
Cermet inserts are commonly produced by powder metallurgical methods including milling powders of the hard constituents and binder phase, pressing the powder to form green bodies of desired shape and finally, liquid phase sintering the green bodies. Provided that good wetting is obtained between the liquid and the solid hard phase grains, strong capillary forces are obtained. The action of these forces is to shrink the porous body essentially isotropically, thereby eliminating porosity. The linear shrinkage is typically 15-30%.
Sintering of titanium carbonitride-based cermets is a complex process, which requires precise control of all steps to obtain a sintered body with desired properties. Generally, after dewaxing, the material is heated under vacuum or in an inert atmosphere to 1250-1350.degree. C. to enable deoxidation and denitrification of the material. Further heating to the final sintering temperature and subsequent cooling is normally done under vacuum or in an atmosphere that may contain both inert and reactive gases. Each of the steps influences the properties of the sintered material and must therefore be optimized carefully.
Conventional sintering processes yield sintered material with several drawbacks, such as lack of toughness and wear resistance. The sintered bodies commonly have a concentration of pores in the center and a surface with varying degrees of enrichment or depletion of the binder phase. Various attempts have been made to improve process control by varying the gas atmosphere during sintering.
Sintering in nitrogen (N.sub.2), accomplished in various ways, provides a means to limit denitrification, which is especially useful for cermets with high nitrogen content.
U.S. Pat. No. 4,990,410 discloses a process for producing a cermet by liquid phase sintering in 0.1-20 torr N.sub.2 at temperatures .gtoreq.1300.degree. C. A nitrogen atmosphere is proven useful for modification of the near surface properties of sintered cermet bodies. U.S. Pat. No. 5,059,491 discloses a process for producing a cermet with maximum hardness at a depth between 5 and 50 .mu.m from the surface by liquid phase sintering in N.sub.2 and cooling in a vacuum. U.S. Pat. No. 4,985,070 discloses a process for producing a high-strength cermet, which is accomplished by sintering the material in progressively increasing nitrogen pressure. U.S. Pat. No. 5,145,505 discloses a process for producing a tough cermet with a binder-depleted surface by sintering in 5-30 torr N.sub.2.
Sintering in CO has been found useful for obtaining improved control over the surface of sintered cermet bodies. WO 99/02746 discloses a process for producing sintered bodies without the common binder phase layer of 1-2 .mu.m thickness on the surface by sintering in CO at pressures 1-80 mbar.
Sintering in CO--N.sub.2 mixtures has been attempted to obtain improved properties of sintered bodies. U.S. Pat. No. 5,856,032 discloses a process for producing Ti(C,N)-based cermets by liquid phase sintering in CO--N.sub.2 mixtures. The gas mixture is used to modify the surface zone of the sintered body, down to a depth of 600 .mu.m. The desired composition of the gas mixture is dependent on the nitrogen content of the hard constituents whereas the total pressure needed is determined by the binder content. The sintered bodies thus produced are characterized in that .gtoreq.90% by mass of the Co and/or Ni-binder is present in a surface layer of 0.01-3 .mu.m depth in comparison to the underlying core amounts in all cases.
U.S. Pat. No. 6,017,488 discloses a process for producing sintered cermet bodies with Co binder. Sintering is performed in CO--N.sub.2 mixtures, in which the partial pressures are kept below 20 mbar. The sintered bodies have a unique feature in that they have a macroscopic Co gradient, in which the Co content decreases essentially monotonously from the center of the body to its surface and reaches a Co content at a depth of 0-10 .mu.m from the surface of 50-99% of that in the center.
A series of titanium carbonitride-based alloys with Co binder are disclosed in U.S. patent application Ser. Nos. 09/563,502, 09/563,501, and 09/564,648, filed concurrently herewith. These have superior performance in metal cutting applications, both with and without single or multiple layer wear-resistant coatings of carbides or nitrides of Ti and/or aluminum oxide. They show a unique behavior during sintering, being quite different from conventional cermets with Ni--Co binder. One feature is the high content of Ta, i.e. .gtoreq.2 at %, preferably 4-7 at %, which increases the nitrogen activity in the material during sintering. Another feature is the optimization of the raw materials that has led to significant improvement of performance in metal cutting. Due to these two features these materials differ substantially from conventional materials and hence they require a sintering process, unlike the ones that are commonly used. If they are sintered according to the processes disclosed in U.S. Pat. No. 6,017,488 or U.S. Pat. No. 5,856,032, they will melt in the conventional way, i.e. from the surface inwards, leading to gas entrapment and unacceptable porosity, which should be avoided in order to fully utilize the potential of these materials.